The present invention relates to an auto-tensioner for imparting a predetermined tension to a belt wound about a pulley.
Conventionally, auto-tensioners have been utilized for maintaining a constant tension of belts wound about a plurality of driven shafts, such as camshafts, of automotive vehicles.
There has been provided an auto-tensioner comprising a pivoting member rotatably supporting a pulley in contacted relation with a belt and pivotally mounted to a stationary shaft, and pivotal-resistance imparting means adapted to utilize frictional resistance or fluid viscous resistance for imparting a pivotal resistance to the pivoting member thereby absorbing vibrational energy of the belt.
FIG. 14 is a sectional view for illustrating an example of a conventional auto-tensioner utilizing the fluid viscous resistance. The auto-tensioner comprises a stationary shaft 101 mounted to a bed 100 and a cylindrical pivoting member 102 pivotally fitted around the outer periphery of the stationary shaft 101 in eccentric to the stationary shaft 101. A pulley 103 in rolling contact with an unillustrated belt is rotatably mounted on the outer periphery of the pivoting member 102 by means of a bearing mechanism 104. A multiple-plate damper 105 as the pivotal-resistance imparting means is disposed within the pivoting member 102. The multiple-plate damper 105 includes first discs 105a and second discs 105b in alternating relation the first disc 105a having the outer periphery thereof locked to the pivoting member 102 whereas the second disc 105b having the inner periphery thereof locked to the stationary shaft 101. A viscous fluid, such as oil or the like, is filled in respective spaces between the first and second discs 105a and 105b. A locking arm 106a projects from the outer periphery of an end of the pivoting member 102, whereas a bracket 106 with an annular base portion is press fitted around the end of the pivoting member. Locked to the arm 106a is an end of a helical tension spring 107, an elastic force of which acts to pivotally bias the pivoting member 102 in a predetermined direction.
According to this auto-tensioner, the multiple-plate damper 105 is adapted to impart the pivotal resistance to the pivoting member 102 for absorbing the vibrational energy of the belt. Additionally, the elastic force of the helical tension spring 107 acts to press against the belt at a predetermined pressure.
In this auto-tensioner the bracket 106 for locking the helical tension spring 107 follows the behavior of the belt to repeat a pivotal motion in conjunction with the pulley 103 and the pivoting member 102. Accordingly, the use of the auto-tensioner over an extended period of time results in the occurrence of a relative rotation between the pivoting member 102 and the bracket 106 press fitted therearound. Such a relative rotation causes a positional shift of the bracket 106 to vary the tensile force of the helical tension spring 107 and hence, a proper tensile force cannot be imparted to the belt.
In order to improve the follow-up characteristic of the pivoting member 102, the pulley 103 and the like to the belt, the reduction of the weight thereof is particularly effective. Unfortunately, if the pivoting member 102 is formed of a light metal, such as aluminum or the like, to serve this purpose, the pivoting member 102 suffers a decreased bonding strength with the bracket 106. This leads to a heavy relative rotation between the pivoting member and the bracket.
FIG. 15 is a sectional view for illustrating an example of the conventional auto-tensioner utilizing the frictional resistance.
In this auto-tensioner, a cylindrical pivoting member 112 having a side plate 112a at one end thereof is pivotally fitted around the outer periphery of a stationary shaft 111 mounted to a bed 100. The pivoting member 112 is in eccentric relation with the stationary shaft 111 and axially movably fitted with the stationary shaft 111. A pulley 113 in rolling contact with an unillustrated belt is rotatably mounted on the outer periphery of the pivoting member 112 by means of a bearing mechanism 114. The pivoting member 112 contains a helical torsion coil spring 115, one end of which is locked to the pivoting member 112 and the other end of which is locked to the bed 100. As deformed by torsion and compression, the helical torsion coil spring 115 is interposed between the side plate 112a of the pivoting member 112 and a spring receiving member 116 at the stationary shaft 111. A frictional member 117 is interposed between the side plate 112a of the pivoting member 112 and the bed 100. Interposed between the fitting surfaces of the stationary shaft 111 and the pivoting member 112 is a bushing 118, which is integrally formed with the frictional member 117.
According to the auto-tensioner of the above construction, the compressional resistance of the helical torsion coil spring 115 acts to press the pivoting member 112 against the bed 100 via the frictional member 117. This imparts a predetermined pivotal resistance to the pivoting member 112 for absorbing the vibrational energy of the belt. Furthermore, a torsional resistance of the helical torsion coil spring 115 causes the pivoting member 112 to pivot about the stationary shaft 111 for pressing the belt at a predetermined pressure.
Unfortunately a problem exists with the above auto-tensioner that the pivoting member 112 tends to incline relative to the axis of the stationary shaft 111 because of a load applied by the belt B. Thus, a poor fitting precision for the stationary shaft 111 and the pivoting member 112 may lead to a significant inclination of the pulley 113, which, in turn, causes disengagement of the belt from the pulley 113. Accordingly, the stationary shaft 111 and the pivoting member 112 must be machined with high precision to secure the fitting precision for the both. Consequently, the machining of the stationary shaft 111 and the pivoting member 112 is cumbersome, resulting in increased fabrication costs.
Additionally, the frictional member 117 is apt to wear particularly on its side to receive the inclined pivoting member 112. On the other hand, an unbalanced load tends to affect the bushing 118 interposed between the fitting surfaces of the stationary shaft 111 and the pivoting member 112 and hence, a local wear is likely to occur. Accordingly, a need exist for the use of an expensive, highly wear resistant super engineering plastic material, such as polyether etherketone (PEEK), polyether sulfone (PES) or the like, as the material for the frictional member 117 and the bushing 118. This further increases the fabrication costs for the auto-tensioner.